True Solar Power

For over a century it was not known how the sun could maintain its temperature and energy output for the billions of years that geologists knew the earth had been in existence. In 1939, Hanz Bethe finally discovered the nuclear fusion cycle that powers the sun, which involves proton-proton fusion.

The idea of using fusion in a weapon dates back as early as the Manhattan project, and was developed primarily by Edward Teller and Stanislaw Ulam. Their concept was to use the x-ray energy produced by an atomic fission device to flow over to a physically separate fusion component and implode and ignite this separate component. This worked on the very first try in the Ivy Mike test of 1952, which yielded 10 megatons of energy. The fusion plasma was inertially confined and produced enormous fusion gain. The physical principle of high-gain inertial confinement fusion was conclusively demonstrated here.

Theodore Maiman operated the first laser on May 16th 1960 at Hughes Research Laboratory using flashlamps and a ruby rod. The initial paper was rejected by Physical Review Letters, but was subsequently published in Nature.

Shortly after the laser was invented, scientists realized a laser could be used to provide the temperatures and pressures needed to implode and ignite fusion fuel, without needing an atomic fission bomb. The major difference, however, was that the laser energy available was many orders of magnitude less than what could be provided by a fission bomb, and this led to many challenges.

Nuckolls’ seminal paper in the field of inertial fusion proposed using a laser pulse shaped precisely in time to implode DT fuel and achieve densities 10,000 times solid. It was estimated that if this could be achieved, then ignition could be achieved with only 1 kJ of laser energy. While this turned out to be impossible due to hydrodynamic instabilities and nonlinear laser plasma interaction issues, it began investigation into laser-driven inertial fusion in earnest.

The High Yield Lithium Injection Fusion Energy (HYLIFE) chamber was the first proposal put forward by Livermore for how to contain sequential fusion bursts and convert the energy into heat to drive a steam cycle. The concept utilized two opposing beams to illuminate a sequence of fusion targets at a repetition rate of once per second inside a waterfall of lithium contained in a steel chamber. The lithium would contain gigajoule fusion bursts and protect the steel chamber from all ions, target debris, and 14 MeV neutrons. Xcimer’s approach is largely based around this original concept.

The Shiva laser was a 10 kJ system at 1 micron wavelength. It was the largest laser in the world at the time, and was built to better understand the coupling of laser energy to a fusion target as well as target implosions. A major result was the discovery of the need to move to shorter wavelengths due to laser plasma interactions inhibiting effective coupling of laser energy to targets.

The first excimer laser was built at Livermore by Paul Hoff in 1972, and by the late 70s many more excimers had been discovered, including argon fluoride and krypton fluoride. Much of the gas chemistry of these lasers had been worked out by the end of the decade, with several kJ-scale systems built.

The Nova laser was built after lessons learned on Shiva, and could hit a target with a 45 kJ laser pulse at a much shorter wavelength of 351 nm. Ignition was expected, but it was more difficult to achieve the needed high-compression implosions than originally thought (the lower the laser energy, the higher the compression of the fuel capsule that is needed). Nevertheless, Nova significantly advanced the science of inertial fusion, and paved the way for the NIF.

A series of underground nuclear tests were fielded to lay to rest the viability of imploding and igniting laboratory scale fusion fuel capsules. These capsules were driven by siphoning a very small amount of x-ray energy from adjacent nuclear detonations, and they achieved ignition and high gain. Detailed reports are classified, but these tests conclusively demonstrated inertial fusion energy is possible with a high-enough energy laser.

The National Academy of Sciences held a review of inertial fusion in 1986, after analysis of all data to date from both laser fusion facilities as well as the Halite Centurion tests. The committee recommended the construction of a 10 MJ laser to ensure robust fusion ignition and high gain.

The National Ignition Facility was sold to Congress with a laser energy of 2 MJ, significantly short of the 10 MJ level recommended by the NAS. The 2 MJ system was a compromise, as a 10 MJ system with then-proposed technologies would have been far too expensive.

During the High Average Power Laser (HAPL) program, the Naval Research Laboratory (NRL) significantly advanced excimer laser technology for inertial fusion energy. In particular, the kJ-scale Electra laser operated repetitively at five shots per second for up to a day at a time, demonstrating key technology necessary for enabling an excimer laser to continuously shoot fusion fuel capsules in a power plant.

In 2022, the NIF achieved scientific breakeven, with the fusion burst from a fusion target exceeding the total laser energy delivered to the target. Since then, the NIF has improved over this result, with 5 MJ of fusion energy produced from a 2 MJ laser pulse. The significance of this result is the production of a fully ignited and burning plasma, with a fuel capsule gain of 20. The data from the NIF fully supports the original 1986 NAS conclusion that a 10 MJ would allow for robust ignition and high gain.